In recent years, secondary batteries have been important power source parts in advanced-function mobile electronic devices. Among such secondary batteries, Li ion secondary batteries, which provide high energy density by virtue of high voltage attained by an appropriate combination of a cathode active material and an anode active material, have replaced conventional NiCd cells and Ni hydrogen cells and now predominantly serve as secondary batteries. However, Li ion secondary batteries containing, in combination, a lithium cobaltate (LiCoO2) cathode active material and a carbon-based anode active material (mainly graphite), the combination being typically employed in current Li ion batteries, cannot provide sufficient consumption power for recent advanced-function and high-load electronic parts. Thus, such Li ion secondary batteries fail to attain performance required of power sources in mobile devices.
Generally, the theoretical electrochemical specific capacity of cathode active material is low. In addition to lithium cobaltate, currently employed lithium manganate and lithium nickelate and lithium iron phosphate, whose practical use is now under investigation, attain a specific capacity lower than the theoretical specific capacity of current carbon-based anode active material. Meanwhile, the performance of the carbon-based anode active material has been enhanced year by year, and the specific capacity thereof becomes nearly equivalent to the theoretical specific capacity. Therefore, the combination of a current cathode active material and a current anode active material is not expected to attain drastic enhancement in power capacity. Thus, current cathode active materials have a limitation in relation to meeting further demand for adding higher functions in electronic device and long-term operation of mobile devices and to adaptation to industrial uses (e.g., electric tools, uninterruptible power sources, and electrical storage devices) and electric vehicles.
Under such circumstances, replacement of a carbon (C)-based anode active material by a metallic anode active material is studied for drastically increasing electric capacity from the currently attained level. Specifically, an anode active material containing a germanium (Ge) substance, a tin (Sn) substance, or a silicon (Si) substance is used, the material having a theoretical specific capacity which is several times to ten times that of a currently utilized carbon-based anode active material. Particularly, Si is most intensively studied, since Si has a specific capacity equivalent to that of metallic Li, which is thought to be difficult to use in practice.
However, since the cathode active material has low specific capacity, the high theoretical specific capacity of Si is not fully utilized for producing actual batteries. A layered or tunnel-form complex oxide, which is currently studied for practical use as a cathode active material and which can serve as a Li intercalation host, has a theoretical specific capacity (per unit mass) capacity slightly higher than 150 mAh/g, which is ½ or less the specific capacity of a current carbon-based anode active material and 1/20 or less the theoretical specific capacity of Si. Therefore, studies must be conducted on a material system for increasing the capacity of cathode active material. One candidate for a new cathode active material is a lithium transition metal silicate compound, which is expected to provide a capacity higher than 300 mAh/g (twice current level) by employing a certain component, and studies thereof have started (see, for example, Patent Document 1 and Non-Patent Document 1).